Sealed reed switch



March 17, 1964 w B, ELLWQQD 3,125,650

SEALED REED SWITCH Filed Aug. 3, 1961 Fla.

ll/l/i I I 1.:

VIII/II, 'IIIIl/Il IN [/5 N TOR AT TORNE Y United States Patent3,125,650 SEALED REED SWITCH Walter B. Ellwood, New York, N.Y., assignorto Bell Telephone Laboratories, Incorporated, New York, N.Y.,. acorporation of New York Filed Aug. 3, 1961, Ser. No. 129,130 1 Claim.(Cl. 200-87) This invention relates to circuit controlling devices andmore particularly to sealed reed type switches or relays of the generaltype disclosed in the patent of Ellwood 2,289,- 830, granted July 14,1942.

This general type of switch usually comprises a hermetically sealedenvelope surrounded by a signal coil and enclosing a flexibleferromagnetic reed which partially overlaps a second ferromagnetic reedand is separated from it by a small gap. When the signal coil isenergized, magnetic Signal fiux flows longitudinally along the two reedsacross the gap and causes their overlapping ends to make contact. Whenthe device is used as a transfer switch, the flexible reed, or swinger,is normally biased against a back contact while the other reedconstitutes a front contact which is engaged only when the signal coilis energized.

One of the primary advantages of the Ellwood switch is its simplicity,and the widespread use of any embodi- "inent thereof is dependentlargely on its susceptibility to mass production techniques. Detractingfrom the usefulness of transfer sealed reed switches is the problem ofproviding a constant predetermined mechanical bias force of the flexibleswinger reed against the back contact. The elastic qualities of theswinger reed, or any spring that biases it, have been found to varyappreciably from one switch to another. Associated with this problem arethefabricatiori difficulties involved in aligning and spacing thevarious elements, especially in more refined forms of j the sealed reedswitch. Further'it has been found that under some circuit conditions aredischarges tend to persist when a large potential'diiference existsbetween the swinger reed and either the front or back contact.

Some of these drawbacks can be obviated by using a conductive permanentmagnet as a back contact member. It has been found, however, that theflux produced by the permanent magnet of devices of the prior art tendsto interfere with the flux produced by the signal coil and therebydegrade the general performance of the device. Moreover, the signal coilflux tends to alter the magnetization of the permanent magnet.

It is an object of this invention to simplify the structure andfacilitate the manufacture of sealed reed transfer switches.

It is another object of this invention to provide a predetermined andreadily controllable contact force between the swinger reed and the backcontact of a sealed reed switch without interfering with the fluxproduced by the signal coil.

These and other objects of my invention are attained in a sealed reedswitch of the general type described above. According to one aspect ofthe invention, a permanent magnet that is located adjacent theoverlapping end of the 1 swinger reed constitutes a back contact forconducting current from the swinger reed when the swinger reed isdisengaged from the front contact. The bias exerted by i the permanentmagnet on the swinger reed can be easily and precisely controlled aswill be explained hereafter.

It is a feature of this invention that the magnetic axis 'reed thatdecreases rapidly with distance.

3,125,650 Patented Mar. 17, 1964 ice tween the north and south poles ofthe permanent magnet be parallel to the broad axis of the swinger reed.This is advantageous for two reasons. First, most of the permanentmagnet return flux is at substantially right angles with the electricfield between the swinger reed and either the front or back contacts.

As a result, most of the gas ions in the gap are restrained fromtraveling directly between the reed and either of the contacts by thecrossed-field current condition in the gap, and the formation of an arcdischarge is retarded. Secondly, the fact that the permanent magnetoperates on a swinger reed through two magnetic gaps results in amagnetic force on the swinger Consequently, there is only a slight forceexerted by the magnet on the swinger reed when it is in contact with thefront contact. On the other hand when it is in contact with thepermanent magnet back contact, the magnetic force on .the swinger reedis sufficiently high to insure a good contact and prevent any accidentalclosure against the front contact.

' These and other objects and features of my invention panying drawing,in which:

FIG. 1 is a sectional view of a sealed reed switch which illustrates theinvention; and

FIGS. 2a and 2b are sections taken along the lines 2-2 of FIG. 1, whenthe swinger reed is biased against the front contact and back contactrespectively.

Referring now to FIG. 1, there is shown a sealed reed transfer switch 10comprising a hermetically sealed glass envelope 11 in which are locatedtwo conducting members 12 and '13 at one end of the envelope. A swingerreed 14 is secured to the other end of the envelope such that each faceof the reed overlaps and is adjacent to a conducting member. A signalcoil 15 encompasses the glass envelope and controls the contact ofswinger reed 14 with conductors 12 and 13.

A permanent magnet 17 is supported at its magnetically neutral point byconductor 12 and has its magnetic axis perpendicular both to the coilaxis and the gap between the reeds. It serves as a back contact forswinger reed 14. In the normal unoperated condition of the switch asshown in FIG. 1, current flows through swinger reed 14, magnet 17, andconductor 12.

Conductor 13 and reed 14 are made of soft magnetic material such as 52alloy, for example. When signal coil 15 is energized, flux flows throughswinger reed 14 and ferromagnetic conductor 13 as illustrated by path HAs is Well known, this signal flux exerts a magnetic attractive forcebetween reed 14 and conductor 13. This force overcomes the magneticforce of magnet 17, and reed 14 virtue of its inherent elasticity andthe field of the magnet 3 and it is held in firm electrical contact withconductor 12 by the magnetic force of magnet 17. Magnet 17 is made ofhard magnetic material which presents a high reluctance path to flux Hso that very little flux is shunted through conductor 12 even if thatconductor is made of magnetic material.

FIG. 2a which is a section taken along lines 2-2 of FIG. 1, shows theposition of swinger reed 14 in its operated condition, that is, whensignal flux flows through the reed and ferromagnetic conductor 13. Inthis view, the signal flux H is designated by crosses as coming out ofthe paper. The polarity of permanent magnet 17 is shown, which resultsin a permanent magnet flux path H It is to be noted that the magneticaxis and hence the flux path H of magnet 17 is at right angles with thesignal flux path H There is therefore no significant component of thepermanent magnet fiux which can have any additive or substractive effecton the signal flux. As a result, the device is equally responsive toflux flowing into the paper as it is to flux flowing out of the paper asshown in FIG. 2a. The operation of switch 10 is therefore independent ofthe direction of signal current flowing in coil 15, and, unlike similardevices of the prior art, is magnetically neutral. With reference toFlIG. 1, the device would be equally responsive to flux flowing fromleft to right as from right to left, as shown.

FIG. 2b illustrates the position of reed 14 in its normal unoperatedcondition. It can be seen that the flux of magnet 17 reaches reed 14 byway of two magnetic gaps, one adjacent the north pole, and one adjacentthe south pole. It can be shown that the magnetic force acting on a bodythrough two gaps decreases much more rapidly with respect to distancethan a corresponding force which acts through only one gap. Therefore,when reed 14 is in its operated position as shown in FIG. 2a, magnet 17exerts a smaller magnetic force on it than would 'be the case if theforce only acted through one magnetic gap.

There is (under some circuit conditions) a potential difference betweenreed 14 and conductor 13 when the reed is transferred from 13 to magnet17 as shown in FIG. 2b or conversely. The resulting electric field Etends to ionize gas molecules in the gap between them which, in theabsence of flux H could cause an arc discharge. However, since flux H isat right angles to electric field E, a crossed field current conditionexists in the gap which forces the ions to follow cycloidal ortroohoidal paths at right angles to both the electric and magneticfields. Hence, many ions and free electrons are prevented fromtraversing the gap directly and undesirable glow or are discharges arereduced in duration if not prevented entirely. This provides a magneticblowout effect similar to that used in large circuit breakers.

Consider next the method of manufacturing the sealed reed switch ofFIG. 1. Envelope 11 is initially in the form of a glass tube that isopen at both ends. Conductors 12 and 13 are mounted in a jig which holdsthem at a slight incline and in contact at the magnet end. The magnetend is then inserted into one open end of the glass tube which issupported in a suitable heating coil to form the seal. A chisel-shapedshim of predetermined thick ness is inserted between back contact 17 andfront contact conductor 13 to maintain the correct spacing and alignmentwhile the end of the tube is being sealed. After the seal is formed andreturned to room temperature the shim is withdrawn.

The subassembly is then inserted between the poles of an electromagnetcapable of producing a high magnetic field, as for example, 25,000gauss. The axis of the permanent magnet is held parallel to the field.Permanent magnet 17 is made of a hard magnetic material having a highcoercive force while conductive 13 is made of soft magnetic materialhaving a low coercive force. When the magnetic field is removed,permanent magnet 17 therefore retains a strong permanent magnetization,

while conductor 13 does not. Permanent magnet 17 is preferably made of aheat treated alloy of 77% platinum and 23% cobalt, while conductor 13and swinger reed 14 are made of 52 alloy but a heat treated alloy of 49%cobalt, 49% iron, and 2% vanadium, may be used to advantage. In the caseof very small short magnets used in miniature versions of the switch itmay be desirable to make conductor -12 of two pieces, one of 52 alloy toseal into the glass and the other of stainless steel or advance wire toprovide a non-magnetic support for the magnet. This may be requiredbecause of difficulty in attaching the conductor 12 to a very smallmagnet at its neutral point.

After the magnetization process, the swinger reed 14 is inserted in thegap between the magnet *17 and the front contact portion of conductor13. The tube assembly with the projecting swinger reed is hung open enddown in the heat coil. While forming gas is blown into the open end ofthe envelope, the second seal is made at the swinger reed end of theenvelope 11 with the swinger reed in a completely unstressed condition.

The foregoing method of manufacture is advantageous for several reasons.Magnet 1-7 is magnetized only after it has been welded to conductor '12and the contact end of the envelope has been sealed. The heat generatedduring the welding and sealing operations therefore cannot affect themagnetization of the magnet. Further, magnetic dirt cannot accumulate onmagnet 17 before it is magnetized; particles can be readily freed bycompressed air from the magnetically inert magnet 17 before it ismagnetized. 'Finally, the back contact bias of magnet 17 on swinger reed14 is readily adjustable over wide limits by the simple expedient ofadjusting the strength of the magnetizing field during the magnetizingoperation.

It should be pointed out that there is only one mechanical spacing gapwhich is to be determined, that is, the gap between magnet 17 andconductor 13 when the contact end of the envelope is sealed. As thisseal is being made, the accompanying heat relaxes the pressure on theshim which defines the gap, so that when the shim is withdrawn there isno shifting of either the magnet or conductor 13.

As pointed out above, the swinger reed 14 is sealed in envelope 11 in anunstressed condition. This is impontant because, as a result, the backcontact of reed 14 against magnet 17 is not dependent on the elasticityof the reed, as in certain prior devices.

In summary, it can be appreciated that the provision of a permanentmagnet .17 having its magnetic axis transverse to the path of signalflux, for biasing the swinger reed 14, is advantageous both because itimproves the operation of the device and because it expeditesmanufacture of the device. A particular advantage is the unvariableresponse of the device which is independent of the direction of currentflow through the signal coil. It is intended, however, that theabove-described device he only illustrative of my inventive concept.Various other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

A magnetically neutral sealed reed switch comprising: first and secondferromagnetic conductors that are substantially parallel and haveoverlapping ends that are normally displaceable from each other; asignal coil surrounding the conductors which produces a magnetic fluxalong the conductors in response to a signal current, whereby theoverlapping ends of the conductors are forced to make contact; a thirdconductor; means for maintaining electrical contact between the thirdconductor and the second ferromagnetic conductor when the signal coil isnot energized, said last-mentioned means comprising a permanent magnetthat is attached to the third conductor and in magnetic contact with heoverlapping end of the second ferromagnetic conductor, whereby apredetermined gap is maintained between the first and secondferromagnetic conductors; the magnetic axis of the permanent magnetbeing at substantially right angles both to the magnetic flux producedby the signal coil in 5 both reeds and gap and to any electric fieldsproduced as a result of a potential diflerence between the first andsecond ferromagnetic conductors whereby the magnetic flux from themagnet acts on any ions in the gap between the ferromagnetic conductorswhich are thus pre- 10 vented from forming an arc discharge of longduration.

References Cited in the file of this patent UNITED STATES PATENTSCharbonneau et a1. Oct. 18, Peek Aug. 4, Wilhelm Oct. 6, Shebanow Mar.22, Juptner Oct. 25, Zuerker et al. Aug. 1, Pileiderer et al. Sept. 12,Jacobson Feb. 6,

